ADS-B is an emerging air traffic control system that can augment or even replace conventional radar systems. ADS-B uses conventional Global Navigation Satellite System (“GNSS”) technology and employs relatively simple broadcast communications links. For a given aircraft, precise position information from the GNSS is combined with other aircraft information such as speed, heading, altitude, and flight number. This combined data (collectively “information”) is then simultaneously broadcast to other ADS-B capable aircraft and ground stations or satellite transceivers, which may further relay the information to Air Traffic Control (“ATC”) centers, and/or back to other ADS-B capable aircraft. Typically, an ADS-B system comprises a plurality of interconnected ground stations for receiving and re-broadcasting information regarding individual aircraft or planes.
As noted, and as shown in FIG. 1, in an ADS-B system information about the location and other “discretes” (e.g., speed, heading, altitude, etc.) of planes (known as “targets”) may be collected by multiple ground stations. The information may be gathered from transmissions received directly from of a target itself (when the target has the necessary equipment) or from other surveillance systems such as legacy radars. The ground stations exchange the information through terrestrial or radio links and then the ground stations broadcast messages about the current target position and discretes to ADS-B capable aircraft (known as “customers”).
For the system to perform effectively, it is critical for customers to receive up-to-date and timely broadcasts about targets. However, the ADS-B broadcast spectrum is very crowded, resulting in increased interference and overall lower quality of reception for customers.
The current state of the art with respect to ground station message broadcasting is described in several patents assigned to Rannoch Corporation, including U.S. Pat. No. 6,567,043 B2, U.S. Pat. No. 6,633,259 B1, and U.S. Pat. No. 6,806,829 B2. These patents describe a technique whereby a system sends to each customer broadcasts through a ground station with the best reception at the customer. Such a ground station may be in the line of sight of the customer, may have the best probability of reception at the given customer, or may simply be the closest to the customer.
A significant shortcoming of the broadcast scheduling described in these patents is the potential for a high level of broadcast duplication. More specifically, with reference to FIG. 1, suppose ground station 110a has the best reception at customer 105a, while ground station 110b has the best reception at customer 105b, but station 110b can be received by customer 105a. In the prior art scheme, both ground stations 110a and 110b broadcast the same message. Given, for example, a crowded airport space and the operation of existing ADS-B message broadcasting techniques, the level of duplication might be quite high, thus decreasing the overall quality of air traffic communications.
There is therefore a need to improve ADS-B infrastructure, and particularly the infrastructure related to ground station message transmissions or broadcasts.